The complement system, as a series of chemical reactions, within the immune system aids in the removal of pathogens from an organism providing an early acting mechanism for the initiation and activation of inflammatory response to microbial infection and other acute insults. While complement activation provides a valuable first-line defense against potential pathogens, the activities of complement that promote a protective inflammatory response can also represent a potential threat to the host. For example, neutrophils are activated by C3 and C5 proteolytic products, and are indiscriminate in their release of destructive enzymes possibly causing organ damage. Additionally, host cell lysis may result from complement activation causing the deposition of lytic complement components on microbial targets as well as host cells nearby.
There are implications that the complement system contributes to pathogenesis of numerous acute and chronic disease states, including septic shock, capillary leakage following thermal burns myocardial infarction, post cardiopulmonary bypass inflammation, transplant rejection revascularization following stroke, ARDS, reperfusion injury, rheumatoid arthritis, multiple sclerosis, myasthenia gravis, and even Alzheimer's disease. Although for nearly all of these conditions, complement is not the cause, it is one of several factors involved in pathogenesis and could be a significant pathological mechanism that offers an effective point for clinical control. The need for effective complement inhibitory drugs is signified by growing recognition of the importance of complement-mediated tissue injury in a variety of disease states. Despite this, currently there is a complete absence of approved drugs for human use that specifically target and inhibit complement activation.
The complement system can be activated through three distinct pathways: the classical pathway, the lectin pathway, and the alternative pathway. The classical pathway is most often triggered by an antibody bound to a foreign particle (i.e., an antigen), therefore requiring previous exposure to that antigen for the generation of a specific antibody. So, while the classical complement pathway, as part of the acquired immune system, typically requires antibodies for activation, the alternate pathway can be activated by C3 hydrolysis or antigens without the presence of prior exposure to antibodies.
The binding of a specific recognition molecule C1q, to antigen-bound IgG and IgM is the first step in activation of the classical pathway. Upon binding of C1q to an immune complex, autoproteolytic cleavage of C1r is followed by C1r activation of C1s, which then acquires the capacity to cleave C4 and C2. C4 cleaves into two fragments, C4a and C4b, which allows the C4b fragments, to form covalent bonds with adjacent hydroxyl or amino groups and the subsequent generation of C3 convertase (C4b2b) through non-covalent interaction with the C2b fragment of activated C2. C3 convertase (C4b2b) activates C3 leading to generation of the C5 convertase (C4b2b3b) and formation of the membrane attack complex (C5b-9) that can cause lysis. The activated forms of C3 and C4 (C3b and C4b) are covalently deposited on the foreign target surfaces, which are recognized by complement receptors on multiple phagocytes.
For the activation of the complement system by way of the lectin pathway, the first step is also the binding of particular recognition molecules, then followed by the activation of associated serine proteases. The lectin pathway uses a protein similar to C1q of the classical complement pathway, and allows binding on multiple pathogens. However, rather than the binding of immune complexes by C1q, the recognition molecules in the lectin pathway are carbohydrate-binding proteins (mannan-binding lectin MBL, and Ficolins). MBL is a calcium-dependent lectin that can initiate the complement cascade by binding carbohydrates to pathogen surfaces.
During inflammation, the expression of MBL is up regulated and L-ficolin is present in serum at similar concentrations as MBL. Therefore, the L-ficolin arm of the lectin pathway is potentially comparable in strength to the MBL arm. Human MBL forms a specific and high affinity interaction through its collagen-like domain with unique C1r/C1s-like serine proteases, termed MBL-associated serine proteases (MASPs). C3b is the protease responsible for activating C4 and C2 to generate the C3 convertase, C4b2b. The mannan-binding lectin pathway is widely thought to have a role in host defense against infection. It has been noted that such patients display substantial increased susceptibility to recurring infections. Other studies have implicated the classical and alternative pathways in the pathogenesis of ischemia/reperfusion injury and the role of the lectin pathway in this disease remains controversial.
The alternative pathway begins the biochemical cascade by spontaneous activation triggered by foreign or other abnormal surfaces including bacteria, damaged tissue, or virally infected cells. In order for the alternative pathway to function there are four plasma proteins required that are directly involved in the alternative pathway and include C3, factors B and D, and P. Proteolytic cleavage of C3b from native C3 is required for the cascade of the alternative pathway to function. C3 belongs to a family of proteins along with C4 and α-2 macroglobulin, which contain a rare posttranslational modification known as a thio-ester bond. Known as the thioester group, they are composed of a glutamine whose terminal carbonyl group is bound to the sulfhydryl group of a cysteine three amino acids away. The bond created is generally unstable and allows the electrophilic carbonyl group of glutamine to form a covalent bond with other molecules via hydroxyl or amino groups. The thioester bond is reasonably stable when isolated within a hydrophobic pocket of intact C3. However, the proteolytic cleavage of C3 to C3b results in the release of a high energy and therefore highly reactive thioester bond on C3b. This mechanism causes C3b to covalently attach to a target as well as an additional molecule of C3a also is released.
In combination with the acknowledged role of thioester in covalent attachment of C3b to complement targets, the C3 thioester is also thought to have a pivotal role in triggering the alternative pathway. The cascade of the alternative pathway also provides a powerful amplification loop for the lectin and classical pathway C3 convertase (C4b2b) since any C3b generated can participate with factor B in forming additional alternative pathway C3 convertase (C3bBb). This implies that inhibition of C3b function will inhibit all three pathways to complement system activation. The alternative pathway C3 convertase is stabilized by the binding of properdin extending the C3 convertase half-life six to ten fold. Addition of C3b to the C3 convertase leads to the formation of the alternative pathway C5 convertase.
Each of the three pathways (classical, lectin and alternative) have been thought to converge at C3, which is cleaved to form products with multiple pro-inflammatory effects. The C5 convertase cleaves C5 to release the most potent anaphylatoxin, C5a. This induces alterations in smooth muscle and vascular tone, as well as vascular permeability and is also a powerful chemotaxin and activator of both neutrophils and monocytes. Inflammatory responses can be substantially amplified by C5a-mediated cellular activation through the induction of the release of multiple additional inflammatory mediators, including cytokines, hydrolytic enzymes, arachidonic acid metabolites and reactive oxygen species. C5 cleavage also generates C5b initiating the formation of C5b-9 known as the membrane attack complex (MAC). In addition to its role as a lytic pore-forming complex, there is strong evidence that the depositing of sublytic MAC may play an important role in inflammation.
Based upon the available clinical and research data, it appears that in most acute and chronic settings, production of C3a and C5a complement activation is mediated by the activation of the complement pathways. Because in clinical settings, both C3a and C5a have been independently shown to be involved, developing suitable methods of inhibition for all pathways would be highly desirable. Both anaphylatoxins C3a and C5a are known to activate leukocytes and platelets. A frequent indicator of cellular activation is the cellular expression of CD11b on leukocytes, and CD62P on platelets. The release of several inflammatory molecules is triggered by the platelet-leukocyte binding mediated by these activation markers. One result of such conjugate formation is the removal of platelets from the circulation, a phenomenon that can contribute to the development of thrombocytopenia.